1. Field of the Invention
The present invention is directed generally to transmitter identification and, more particularly, to a system and method for the identification of transmitters using limited information.
2. Description of the Related Art
Existing position location technologies based on global positioning system (GPS) use a network of satellites in the sky which transmit signals at a known time. A GPS receiver on the ground measures the time of arrival (TOA) of the signals from each satellite it can detect. The TOA of the signals from the satellites, along with the exact position of the satellites and the exact time the signal was transmitted from each satellite is used to triangulate the location of the GPS receiver. A typical GPS receiver requires four satellites to make a triangulation, and the performance of the resulting calculation increases as the number of satellites that can be detected increases.
In an alternative to, or augmentation of, GPS, an existing network of cellular base stations can be treated as a network of satellites for purposes of location determination. Similar to GPS technology, the exact position of each base station, the exact time at which the base station is transmitting a signal, and the TOA of the base station signals at a mobile unit can be used to triangulate the location of the mobile unit. This technique is described by some service providers as Advanced Forward Link Trilateration (AFLT). Wireless networks may also be used in conjunction with GPS to determine the location of the mobile unit.
A significant problem faced by the mobile station is to measure the TOA of the signals that are received from each base station. Different wireless technologies may take different approaches to TOA measurements. Code division multiple access (CDMA) is one such technology. CDMA modulation is one of several techniques that allow a large number of system users to share a communication system. It is possible to utilize measurements of conventional CDMA modulation techniques to determine the location of a mobile unit using AFLT techniques.
CDMA modulation techniques are disclosed in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. The above-referenced patent discloses the use of a phase-coherent and chip-synchronous chip sequence that is defined as a pilot chip sequence, or pilot signal. The pilot signal can be used to provide phase and time acquisition and tracking, and multi-path correction.
Methods for acquiring the pilot signals are disclosed in the above-referenced patent and in the following patents: (1) U.S. Pat. No. 5,781,543, issued on Jul. 14, 1998 and entitled “POWER-EFFICIENT ACQUISITION OF A CDMA PILOT SIGNAL;” and (2) U.S. Pat. No. 5,805,648, issued on Sep. 8, 1998 and entitled “METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,” both of which are assigned to the assignee of the present invention and the disclosures thereof are incorporated herein by reference.
When the mobile unit is first powered on, it must establish a communication link with a base transceiver station (BTS). The mobile unit will typically receive pilot signals from a plurality of BTSs. The mobile unit will search for the signals from the BTSs and will establish a communication link with a selected BTS to permit the reception and transmission of data, such as audio signals, over the established communication link. The selection of a particular BTS and the actual process of communication between the mobile unit and the selected BTS are well known in the art and need not be discussed in detail herein.
As discussed in the above-referenced patents, each BTS periodically broadcasts the same pseudo-noise (PN) code pilot signal, but with a different time offset. That is, each BTS transmits the same PN code, but the start of transmission of the PN code from the transmitter in each BTS is delayed in time by a precisely known offset. The time offsets are measured in multiples of 64 chips. As those skilled in the art will appreciate, a “chip” is a single piece of data in the PN sequence. Because the data is transmitted at a known rate, chips may be used as a measure of time. Although the present description may be characterized in actual units of time, it is more convenient to refer to the time in terms of chips or portions of chips because the TOA delays due to the PN offset as well as propagation delay measurements may be calculated in terms of chips.
To acquire the pilot signal, the mobile unit must synchronize with the time offset and frequency of the signal transmitted by a BTS. The object of a “searcher” process in the wireless device is to find the time offset of the received signal. The searcher uses an estimated frequency. If the estimated frequency is not sufficiently close to the frequency of the pilot signal, the received signal will not be acquired.
When a BTS is properly detected, the output of the searcher is a pulse, which may be considered a correlation pulse. This correlation pulse may be used to measure the TOA of the signal from the BTS. It is necessary to measure the TOA from a number of BTSs to accurately determine the location of the mobile unit. In a typical embodiment, the TOA from at least four BTSs must be calculated to determine the location of the mobile unit. A more accurate determination may be made if TOA signals are received from additional BTSs.
An accurate determination of the location of the mobile unit requires precise identification of each BTS from which a signal is received as well as the precise time at which signals were transmitted from each BTS. However, the mobile unit is often unable to precisely identify the BTS because only limited information is received from the BTS. That is, the mobile unit does not always receive complete information from each BTS that permit the unique identification of each BTS. In a particular geographic region, multiple BTSs may have the same PN offset resulting in potential ambiguity as to the identification of a BTS from which a signal has been received. Such ambiguity leads to inaccuracies in the location determination process. Therefore, it can be appreciated that there is a significant need for a technique by which transmitters may be identified using the limited information received by a mobile unit. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.